Want to secure an office against wireless hackers? Try smearing the windows with carbon.

Researchers in England have developed a transparent film that blocked 90 percent of radio waves. The material, described February 19 in Scientific Reports, is made from a stack of quartz layered with graphene — a one-atom-thick latticework of carbon atoms — that’s only several atoms thick.

The film could provide wireless security by absorbing unwanted radio transmissions. The next challenge for researchers is tuning the film to a wider range of frequencies and developing a method for making larger sheets.